containerize: update docs to activate env before using container templates (#40493)

lib/spack/docs/containers.rst

@@ -519,6 +519,13 @@ the example below:
    COPY data /share/myapp/data
    {% endblock %}
 
+The Dockerfile is generated by running:
+
+.. code-block:: console
+
+   $ spack -e /opt/environment containerize
+
+Note that the environment must be active for spack to read the template.
 The recipe that gets generated contains the two extra instruction that we added in our template extension:
 
 .. code-block:: Dockerfile